Oxide coated cutting insert

ABSTRACT

The present invention relates to a cutting tool insert comprising a body of a hard alloy of cemented carbide, cermet, ceramics, cubic boron nitride based material or high speed steel a hard and wear resistant coating, comprising one or several layers, at least one of which an (Al,Cr) 2 O 3  layer is applied. This invention is particularly useful for machining of steel and stainless steel. The coating with a total thickness of 2-20 μm comprises one or several layers, at least one of which is an (Al,Cr) 2 O 3  layer with a thickness of 1-5 μm having a corundum phase crystalline structure and a composition (Al 1-y Cr y ) 2 O 3  with 0.4≦y≦0.6. The (Al,Cr) 2 O 3  layer is further characterized having a fiber texture with rotational symmetry in the direction of the coated surface normal with an inclination angle, φ, of the basal planes relative to the coated surface normal or the inclination angle, φ, for the highest peak in the pole plot with 70°&lt;φ&lt;90° as determined by, e.g., electron back scattering diffraction (EBSD) or x-ray diffraction (XRD).

BACKGROUND OF THE INVENTION

The present invention relates to a tool for machining by chip removal comprising a body of a hard alloy of cemented carbide, cermet, ceramics, cubic boron nitride based material or high speed steel and a hard and wear resistant oxide designed to be used in machining of steel and stainless steel, preferably at high cutting speeds. The said coating is composed of one or more layers of which at least one layer is a textured physical vapour deposited (PVD) corundum phase alumina containing chromium (Al,Cr)₂O₃.

Textured α-Al₂O₃ layers, produced with chemical vapour deposition (CVD) are disclosed in, e.g., EP 603144, EP 1528125, EP 1477581, EP 659153, EP 1655387, EP 659903, EP 738336, EP 1655388, EP 1655392, US 2007/104945, US 2004/202877.

EP 1479791 discloses a cutting tool composed of cemented carbide or cermet, and a hard coating; wherein the hard coating includes an α-Al₂O₃ layer formed by CVD, with the highest peak, measuring the inclination of the α-Al₂O₃ basal planes relative to the normal of the surface within a range of 0-10 degrees as determined by electron back scattering diffraction (EBSD).

EP 744473 discloses textured γ-Al₂O₃ layers produced by PVD.

U.S. Pat. No. 5,310,607 discloses a hard coating including (Al,Cr)₂O₃ crystals and a chromium content higher than 5 at % wherein the (Al,Cr)₂O₃ is a single crystal. The coating is deposited at a temperature lower than 900 C. The hard coating is deposited by a CVD or PVD process.

When machining steels with an alumina coated cemented carbide tool, the cutting edge is worn according to different wear mechanisms, such as chemical wear, abrasive wear, adhesive wear and by edge chipping caused by cracks formed along the cutting edge. The domination of any of the wear mechanisms is determined by the application, and is dependent on properties of the machined material, applied cutting parameters and the properties of the tool material. In general, it is very difficult to improve all tool properties simultaneously, and commercial cemented carbide grades have usually been optimised with respect to one or few of the above mentioned wear types, and have consequently been optimised for specific application areas. This can, for instance, be achieved by controlling the texture of the alumina layer.

It is an object of the present invention to provide a wear resistant and hard oxide coated cutting tool with enhanced performance for machining of steel and stainless steel.

The cutting tool insert according to the present invention includes a body of a hard alloy of cemented carbide, cermet, ceramics, cubic boron nitride based material or high speed steel comprising a textured oxide layer of corundum phase (Al,Cr)₂O₃ with excellent metal machining properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows a schematic view of the hexagonal crystal structure with the a-axis (100), b-axis (010) and c-axis (001) marked.

FIG. 1 b shows a schematic view of the fibre texture with (S) coating surface and (φ) inclination angle of the c-axis (001) of the hexagonal structure (FIG. 1 a) and the normal (n) to the coating surface.

FIG. 2 shows a schematic side view of the deposition chamber showing (1) vacuum chamber, (2 a) cathode material A, (2 b) cathode material B, (3) fixture, (4) power supply for biasing, (5 a) cathodic arc power supply (5 b) cathodic arc power supply, (6) inlet for process gas and (7) outlet for vacuum pump.

FIG. 3 shows a scanning electron micrograph in secondary mode of a fractured cross section of a coating according to the invention. (A) body, (B) bonding layer, (C) (Al,Cr)O layer, (D) (Al,Cr)N layer and (E) TiN layer.

FIG. 4 shows an x-ray diffraction pattern of a textured (Al,Cr)₂O₃ layer. The peaks of cemented carbide are marked with solid lines whereas the peaks originating from (Al,Cr)₂O₃ with dashed lines.

FIG. 5 shows (A) (001) pole figure and (B) (001) pole plot graph of a (Al,Cr)₂O₃ layer according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, there is provided a cutting tool for machining by chip removal, particularly useful in metal cutting of steel and stainless steel, comprising a body of a hard alloy of cemented carbide, cermet, ceramics, cubic boron nitride based material or high speed steel onto which a coating is deposited comprising:

preferably a first (innermost) bonding layer (FIG. 3(B)) of, e.g., TiN or (Al,Cr)N preferably less than 0.5 μm according to prior art.

a layer of (Al_(1-y)Cr_(y))₂O₃ with 0.4≦y≦0.6, preferably y=0.5, with a thickness of 0.5-10 μm, preferably 1-5 μm, most preferably 2-4 μm, with textured columnar grains. The (Al,Cr)₂O₃ layer has a corundum structure formed by PVD and a fiber texture with rotational symmetry in the direction of the coated surface normal with an inclination angle, φ, (FIG. 1 b) of the basal planes relative to the coated surface normal (FIG. 5A) or the inclination angle, φ, for the highest peak in the pole plot (FIG. 5 B) with 70°<φ<90°, preferably 80°<φ<90° as determined by, e.g., electron back scattering diffraction (EBSD) or x-ray diffraction (XRD).

Said (Al,Cr)O layer has a compressive stress level of −4.5<σ<−0.5 GPa, preferably of −3.0<σ<−1.0 GPa.

The composition, y, of (Al_(1-y)Cr_(y))₂O₃ is determined by, e.g., EDS or WDS.

Said body may further be coated with an inner single- and/or multilayer coating of, e.g. TiN, TiC, Ti(C,N), (Al,Cr)N or (Ti,Al)N, preferably (Ti,Al)N, (Al,Cr)N, and/or an outer single- and/or multilayer coating of, e.g. TiN, TiC, Ti(C,N), (Al,Cr)N or (Ti,Al)N, preferably (Ti,Al)N, (Al,Cr)N, to a total thickness 1 to 20 μm, preferably 1 to 10 μm and most preferably 2 to 7 μm according to prior art.

The deposition method for the layer of the present invention is based on cathodic arc evaporation of an alloy or composite cathode under the following conditions; (Al,Cr)₂O₃ layers are grown using Al+Cr-cathodes with a composition between (40 at % Al+60 at % Cr) and (60 at % Al+40 at % Cr) and preferably between (45 at % Al+55 at % Cr) and (55 at % Al+45 at % Cr). The evaporation current is between 50 A and 200 A depending on the cathode size and preferably between 60 A and 90 A using cathodes of 63 mm in diameter. The layers are grown in an Ar+O₂ atmosphere, preferably in a pure O₂ atmosphere at a total pressure of 1.0 Pa to 5.0 Pa, preferably 1.0 Pa to 2.5 Pa. The bias is −100 V to −250 V, preferably −125 V to −175V. The deposition temperature is between 300° C. and 550° C., preferably between 350° C. and 450° C.

The invention also relates to the use of cutting tool inserts according to the above for machining of steel and stainless steel at cutting speeds of 75-600 m/min, preferably 150-500 m/min, with an average feed, per tooth in the case of milling, of 0.08-0.5 mm, preferably 0.1-0.4 mm depending on cutting speed and insert geometry.

Example 1

Grade A: Cemented carbide inserts with the composition 10 wt % Co and balance WC, were used.

Before deposition, the inserts were cleaned in ultrasonic baths of an alkali solution and alcohol. The system was evacuated to a pressure of less than 2.0×10⁻³ Pa, after which the inserts were sputter cleaned with Ar ions. At first, a bonding layer of TiN with a thickness of 0.2 μm followed by a textured (Al,Cr)₂O₃ layer of thickness 2.5 μm, were grown by cathodic arc evaporation of an alloyed (50 at % Al+50 at % Cr) cathodes, 63 mm in diameter (position (2 a) and (2 b) in FIG. 2 a) in 99.995% pure O₂ atmosphere at a total pressure of 1.5 Pa and a deposition temperature of about 400° C. to a total coating thickness of 3 μm. The evaporation current was 75 A and the bias was held at −150 V during depositions. Finally, a top colour coating consisting of 0.3 μm (Al,Cr)N and 0.2 μm TiN was applied.

A fractured cross-section SEM micrograph of the coating is shown in FIG. 3 with (A) body, (B) bonding layer, (C) (Al,Cr)O layer, (D) (Al,Cr)N layer and (E) TiN layer.

The XRD patterns of the as-deposited layers were obtained using CuKa-radiation and a θ-2θ configuration. FIG. 4 shows the XRD pattern of a coating according to the invention with a textured corundum phase (Al,Cr)₂O₃ layer. The peaks originating from the (Al,Cr)₂O₃ layer are marked with dotted lines whereas the peaks of cemented carbide are marked with solid lines

The EBSD pole figure (FIG. 5(A)) and pole plot graph (FIG. 5(B)) of the as-deposited corundum phase (Al,Cr)₂O₃ layers in the c-axis (001) direction (FIG. 1 a), respectively, showing a fiber texture (rotational symmetry) in the direction of the coated surface normal (FIG. 1 b) with an inclination angle, φ (FIG. 1 b), of the basal planes relative to the coated surface normal between 70 and 90°. The highest peak in the pole plot is close to 90°. The EBSD data were obtained using a LEO Ultra 55 scanning electron microscope operated at 20 kV equipped with a HKL Nordlys II EBSD detector and evaluated with the Channel 5 software.

The residual stresses, σ, of the (Al,Cr)₂O₃ layer was evaluated by XRD measurements using the sin²ψ method. The measurements were performed using CrKα-radiation on the (Al,Cr)₂O₃ (116)-reflection. The residual stress value was 2.4±0.3 GPa as evaluated using a Possion's ratio of v=0.26 and Young's modulus of E=420 GPa.

The composition, y=0.61, of (Al_(1-y)Cr_(y))₂O₃ was estimated by energy dispersive spectroscopy (EDS) analysis using a LEO Ultra 55 scanning electron microscope with a Thermo Noran EDS detector operating at 10 kV. The data were evaluated using a Noran System Six (NSS ver 2) software.

Example 2

Grade B: A layer of 3.0 μm Ti_(0.34)Al_(0.66)N was deposited by PVD on cemented carbide inserts with the composition 10 wt % Co and balance WC, according to prior art.

Example 3

Grade C: A coating consisting of 3.0 μm Ti(C,N)+3 μm α-Al₂O₃ was deposited by CVD on cemented carbide inserts with the composition 10 wt % Co and balance WC, according to prior art.

Example 4

Grade D: Example 1 was repeated using cemented carbide inserts with the composition 5 wt % Co and balance WC.

Example 5

Grade E: A layer of 3.0 μm Ti_(0.34)Al_(0.66)N was deposited by PVD on cemented carbide inserts with the composition 5 wt % Co and balance WC, according to prior art.

Example 6

Grade F: A coating consisting of 3.0 μm Ti(C,N)+3.0 μm α-Al₂O₃ was deposited by CVD on cemented carbide inserts with the composition 5 wt % Co and balance WC, according to prior art.

Example 7

Grades A, B and C were tested in machining in steel.

Operation Face milling Cutter diameter 125 mm Material SS1672 Insert type SEEX1204AFTN-M15 Cutting speed 300 m/min Feed  0.2 mm/tooth Depth of cut  2.5 mm Width of cut 120 mm Results Tool life (min) Grade A (grade according to invention) 6.1 Grade B 5.9 Grade C 3.4

The test was stopped at the same maximum flank wear. The wear resistance was much improved with the grade according to the invention.

Example 8

Grades A, B and C were tested in machining in stainless steel.

Operation Shoulder milling Cutter diameter   32 mm Material SS1672 Insert type XOEX120408-M07 Cutting speed  275 m/min Feed 0.25 mm/tooth Depth of cut   3 mm Width of cut  8.8 mm Results Tool life (min) Grade A (grade according to invention) 5.1 Grade B 4.1 Grade C 2.3

The test was stopped at the same maximum flank wear. The wear resistance was much improved with the grade according to the invention.

Example 9

Grades D, E and F were tested in machining in stainless steel.

Operation Interrupted turning Material SS2348 Insert type CNMG120408-MR3 Cutting speed  80 m/min Feed 0.3 mm Depth of cut   2 mm Results Tool life (cycles) Grade D (grade according to invention) 5 Grade E 2 Grade F 4

The test was stopped at the same maximum flank wear. The wear resistance was much improved with the grade according to the invention.

Example 10

Grades D, E and F were tested in machining in steel.

Operation Interrupted turning Material SS1672 Insert type CNMG120408-MR3 Cutting speed 350 m/min Feed  0.3 mm Depth of cut  2 mm Results Tool life (min) Grade D (grade according to invention) 9.5 Grade E 4.1 Grade F 9.2

The test was stopped at the same maximum flank wear. The wear resistance was much improved with the grade according to the invention. 

1. Cutting tool insert comprising a body of a hard alloy of cemented carbide, cermet, ceramics, cubic boron nitride based material or high speed steel a hard and wear resistant coating is applied, comprising one or several layers, at least one of which is an (Al,Cr)₂O₃ layer characterised in that said layer has a corundum phase crystalline structure and a composition (Al_(1-y)Cr_(y))₂O₃ with 0.4≦y≦0.6, preferably y=0.5, with a thickness of 0.5 to 10 μm preferably 1-5 μm and a fiber texture, rotational symmetry, in the direction of the coated surface normal with an inclination angle, φ, of the basal planes relative to the coated surface normal is 70°<φ<90° or the inclination angle, φ, for the highest peak in the pole plot is 70°<φ, <90°.
 2. Cutting tool insert according to claim 1 characterised in that said inclination angle, φ, of the basal planes relative to the coated surface normal is 80°<φ<90°.
 3. Cutting tool insert according to claim 1 characterised in that said layer has a residual stress of −4.5<σ<−0.5 GPa, preferably of −3.0<σ<−1.0 GPa.
 4. Cutting tool insert according to claim 1 characterised in that said layer has been deposited with PVD, preferably cathodic arc evaporation.
 5. Cutting tool insert according to claim 1 characterised in that said body is coated with an inner single- and/or multilayer coating of, e.g. TiN, TiC, Ti(C,N), (Al,Cr)N or (Ti,Al)N, preferably (Ti,Al)N, (Al,Cr)N, and/or an outer single- and/or multilayer coating of, e.g. TiN, TiC, Ti(C,N), (Al,Cr)N or (Ti,Al)N, preferably (Ti,Al)N, (Al,Cr)N, to a total coating thickness of 1 to 20 μm, preferably 1 to 10 μm according to prior art.
 6. Method of making a cutting tool insert according claim 1 characterised in depositing the (Al,Cr)₂O₃ layer by cathodic arc evaporation using Al+Cr-cathodes with a composition between (40 at % Al+60 at % Cr) and (60 at % Al+40 at % Cr) and preferably between (45 at % Al+55 at % Cr) and (55 at % Al+45 at % Cr), an evaporation current between 50 A and 200 A depending on the cathode size in an Ar+O₂ atmosphere, preferably in a pure O₂ atmosphere at a total pressure of 1.0 Pa to 5.0 Pa, preferably 1.0 Pa to 2.5 Pa, a bias of −100 V to −250 V, preferably −125 V to −175V and a deposition temperature of between 300° C. and 550° C., preferably between 350° C. and 450° C.
 7. Method of using a cutting tool insert according to claim 1 for machining of steel and stainless steel at cutting speeds of 75-600 m/min, preferably 150-500 m/min, with an average feed, per tooth in the case of milling, of 0.08-0.5 mm, preferably 0.1-0.4 mm depending on cutting speed and insert geometry. 